1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, there are known to be image forming apparatuses that form electrostatic latent images on a photosensitive member by using a rotating polygonal mirror to deflect a light beams emitted from a light source and scanning the photosensitive member with the deflected light beams. This kind of image forming apparatus includes an optical sensor (beam detection (BD) sensor) for detecting the light beams deflected by the rotating polygonal mirror, and the optical sensor generates a synchronization signal upon detecting the light beam. By causing the light beams to be emitted from the light source at a timing determined using the synchronization signals generated by the optical sensor as a reference, the image forming apparatus aligns the writing start positions for the electrostatic latent image (image) in the direction (main scanning direction) in which the light beams scan the photosensitive member.
Also, there are known to be image forming apparatuses that include multiple light-emitting portions (light emitting elements) as a light source for emitting multiple light beams that each scan a different line on the photosensitive member in parallel in order to realize a higher image formation speed and higher resolution images. With this kind of multi-beam image forming apparatus, a higher image formation speed is realized by scanning multiple lines in parallel using multiple light beams, and higher resolution images are realized by adjusting the interval between the lines in the sub-scanning direction.
Japanese Patent Laid-Open No. 2008-89695 discloses an image forming apparatus that includes multiple light-emitting portions (light emitting elements) as a light source and is capable of adjusting the resolution in the sub-scanning direction by performing rotational adjustment of the light source in the plane in which the light-emitting portions are arranged. This kind of resolution adjustment is performed in the step of assembling the image forming apparatus. Japanese Patent Laid-Open No. 2008-89695 discloses a technique for suppressing misalignment in the writing start positions in the main scanning direction for the electrostatic latent image that occurs due to light source attachment errors in the assembly step. Specifically, the image forming apparatus uses a BD sensor to detect light beams emitted from a first light-emitting portion and a second light-emitting portion and generates multiple BD signals. Furthermore, the image forming apparatus sets a light beam emission timing for the second light-emitting portion relative to the light beam emission timing for the first light-emitting portion based on the generation timing difference between the generated BD signals. This compensates for light source attachment errors in the assembly step and suppresses misalignment in the writing start positions for the electrostatic latent image between the light-emitting portions.
However, the following problems are present in the method of measuring the generation timing difference between BD signals generated by the BD sensor as described above, in an optical scanning apparatus (image forming apparatus) including multiple light-emitting portions (light emitting elements) as a light source. Usually, the response speed of the BD sensor when a light beam is incident on the BD sensor changes according to the incident light power. If there is variation in the incident light power, on the BD sensor, of the multiple light beams used for measuring the time interval between the BD signals (BD interval), variation will appear in the measurement result of the time interval between the pulses (BD signals) generated by the BD sensor, and a measurement error can occur. For this reason, when BD interval measurement is to be executed, the incident light power, on the BD sensor, of the multiple light beams used in measurement needs to be made constant.
In order to make the light power of the light beams incident on the BD sensor from the light-emitting portions constant, it needs to execute automatic power control (APC) according to which the light power of the light beams emitted from the light-emitting portions is controlled so as to be a constant light power (target light power). Usually, due to the execution of APC, several milliseconds of time are needed from when the light-emitting portions start emitting light to when the light-emitting portions enter a state of being stable at a constant light power (target light power). For this reason, when BD interval measurement is to be performed, if sufficient time for executing APC on the light-emitting portions used in measurement cannot be reserved, there is a possibility that it will not be possible to stabilize the light power of the light beams emitted by the light-emitting portions at a constant light power and that the above-described measurement error will occur.